System and method of a biosensor for detection of vasodilation

Abstract

An optical circuit detects PPG signals reflected from skin tissue at one or more different wavelengths. A processing circuit integrated in the biosensor or in communication with the biosensor processes the PPG signals to obtain a period of vasodilation, a level of vasodilation and rate of change of the level of vasodilation. The processing circuit compares the level of vasodilation to a normal range and determines an arterial stiffness index using the comparison and the rate of change of the level of vasodilation.

The invention claimed is: 
     
       1. A device, comprising:
 an optical circuit configured to:
 detect a plurality of photoplethysmography (PPG) signals over a measurement period, wherein the plurality of PPG signals include at least a first PPG signal obtained from light at a first wavelength reflected from or transmitted through skin tissue of a user and a second PPG signal obtained from light at a second wavelength reflected from or transmitted through skin tissue of the user; and 
 
 at least one processing device configured to:
 determine at least one vasodilation period using one or more of the first PPG signal or the second PPG signal detected over the measurement period; 
 determine a level of vasodilation or vasoconstriction during the vasodilation period using the first PPG signal and the second PPG signal; 
 compare the level of vasodilation or vasoconstriction to a normal range of levels, wherein the normal range of levels is determined using PPG signals from healthy users in a general sample population; and 
 determine an arterial stiffness index using the comparison, 
 
 wherein determining the level of vasodilation or vasoconstriction includes:
 determining an R value using a ratio of an AC component of the first PPG signal and an AC component of the second PPG signal; and 
 determining the level of vasodilation or vasoconstriction during the vasodilation period using the R value. 
 
 
     
     
       2. The device of  claim 1 , wherein the vasodilation period is a first vasodilation period, and the at least one processing device is further configured to:
 determine a rate of change of the level of vasodilation or vasoconstriction during the vasodilation period; and 
 determine the arterial stiffness index using the comparison and the rate of change of the level of vasodilation or vasoconstriction during a vasodilation period. 
 
     
     
       3. The device of  claim 2 , wherein the at least one processing device is further configured to:
 determine an increase in the arterial stiffness index in response to a lower rate of change of the level of vasodilation or vasoconstriction during the second vasodilation period. 
 
     
     
       4. The device of  claim 1 , wherein the vasodilation period is a first vasodilation period, and the at least one processing device is further configured to:
 determine a correlation between a first pulse shape of the first PPG signal and a second pulse shape of the second PPG signal over the measurement period; and 
 determine the level of vasodilation or vasoconstriction during a second vasodilation period using the correlation between the first pulse shape and the second pulse shape. 
 
     
     
       5. The device of  claim 4 , wherein the at least one processing device is further configured to:
 determine an increase in the level of vasodilation or vasoconstriction during the second vasodilation period in response to an increase in a correlation value between the first pulse shape and the second pulse shape. 
 
     
     
       6. The device of  claim 1 , wherein the vasodilation period is a first vasodilation period, and the at least one processing device is further configured to:
 determine a change in amplitude of a low frequency component in one or more of the first PPG signal or the second PPG signal, wherein the low frequency component is not affected by pulsatile blood flow due to a cardiac cycle; and 
 determine the level of vasodilation or vasoconstriction during a second vasodilation period using the change in amplitude of the low frequency component. 
 
     
     
       7. The device of  claim 1 , wherein the vasodilation period is a first vasodilation period, and the at least one processing device is further configured to:
 determine a change in optical absorption properties of the tissue using the first PPG signal and the second PPG signal; and 
 determine the level of vasodilation or vasoconstriction during a second vasodilation period using the change in the optical absorption properties. 
 
     
     
       8. The device of  claim 7 , wherein the change in optical absorption properties is due to one or more of: an increase in blood flow in the tissue, movement of tissue due to widening of vessels or a change in tissue hue. 
     
     
       9. The device of  claim 1 , wherein the level of vasodilation or vasoconstriction includes a measurement of one or more of: a percentage of change in vessel width, diameter or planar area. 
     
     
       10. The device of  claim 1 , wherein the first wavelength penetrates the skin tissue of the user at a greater depth than the second wavelength. 
     
     
       11. The device of  claim 1 , wherein the at least one processing device is further configured to:
 compare the arterial stiffness index to one or more other measures of arterial stiffness; and 
 average the arterial stiffness index with the one or more other measures of arterial stiffness to determine an updated arterial stiffness index. 
 
     
     
       12. A device, comprising:
 an optical circuit configured to:
 detect a plurality of photoplethysmography (PPG) signals over a measurement period, wherein the plurality of PPG signals include at least a first PPG signal obtained from light at a first wavelength reflected from or transmitted through skin tissue of a user and a second PPG signal obtained from light at a second wavelength reflected from or transmitted through skin tissue of the user; and 
 
 at least one processing device configured to:
 determine an R value using the first PPG signal and the second PPG signal detected during the measurement period, wherein the R value is determined using a ratio of an AC component of the first PPG signal and an AC component of the second PPG signal; 
 determine a level of vasodilation or vasoconstriction during a vasodilation period using the R value; 
 compare the level of vasodilation or vasoconstriction to a normal range of levels, wherein the normal range of levels is determined using PPG signals from healthy users in a general sample population; and 
 determine an arterial stiffness index using the comparison. 
 
 
     
     
       13. The device of  claim 12 , wherein the at least one processing device is further configured to:
 determine a rate of change of the level of vasodilation or vasoconstriction during the vasodilation period; and 
 determine the arterial stiffness index using the comparison and the rate of change of the level of vasodilation or vasoconstriction. 
 
     
     
       14. The device of  claim 13 , wherein the at least one processing device is further configured to:
 determine an increase in the arterial stiffness index in response to a lower rate of change of the level of vasodilation or vasoconstriction during the vasodilation period. 
 
     
     
       15. The device of  claim 12 , wherein the at least one processing device is further configured to:
 compare the arterial stiffness index to one or more other measures of arterial stiffness; and 
 adjusting the arterial stiffness index using the one or more other measures of arterial stiffness to determine an updated arterial stiffness index. 
 
     
     
       16. The device of  claim 12 , wherein the first wavelength is in a range from 650 nm to 1350 nm and the second wavelength is in a range from 380 nm to 649 nm.